I don't buy it. Basic science professors are a fickle bunch. I'm sure he wants the OP to stay in lab and work for a while longer, but I'm not sure he's just bluffing about it. It sounds to me like the OP didn't entirely clear this with him ahead of time, in which case it isn't even that odd of a demand. Unless you've cleared it ahead of time, assuming you're going to leave after four years is a bad idea.
Of course, he knew my intention. Remember, he wrote a so-called LOR (letter of recommendation) for me albeit one that would have killed my dream! Such a jerk: can't believe this guy!